The length and width of wool fabrics usually change in a reversible way when the regain is altered. This has been called hygral expansion. The total reversible change in length or width from dry to saturation depends upon the fabric construction and the finishing treatments and can range from 2% to 12%. Frequently the hygral expansion curve of a worsted fabric exhibits two regions. There is an increase of dimensions with increasing regain, up to about 20% regain, and after passing through a maximum, a decrease of dimensions takes place at higher regains. The reasons for this behavior are discussed. It has been shown that increase and decrease of curvature of single fibre arcs is primarily responsible for the effect at low regains. Fabrics in which the weave crimp is large exhibit a large change of dimensions, and setting treatments give an increased hygral expansion compared with unset cloth. The reasons for these variations with construction and setting are examined. In the high regain region, it seems that some jamming of the structure occurs and as fibres swell further, the dimensions decrease. The forces which are operative in the fabric in both regions of the hygral expansion curve support this idea.